Liquid-filled variable focus lenses wherein one or both lens surfaces are caused to change shape by a change in liquid pressure have been known for over a century. Such lenses have been proposed for use, for example, in spectacles to provide a "reading addition" for people suffering from presbyopia. This application normally involves a composite lens in which a person's distance prescription (if any) is provided by a conventional rigid plastic or glass lens, and a variable additional power is provided by a liquid-filled lens. Three diopters is the theoretical maximum amount of added optical power needed to move the focal point of a spectacle lens from infinity to reading distance.
There have been a number of basic approaches to the design of liquid-filled variable focus lenses over the last one hundred years. In 1893, Ohmart disclosed a "Device for Reflecting and Refracting Radiant Energy" which included a pair of parallel transparent elastic plates or membranes, with the space between them filled with liquid (U.S. Pat. No. 504,890). By pumping liquid into and out of the space between the membranes, the membranes were caused to bow, and hence create a variable power lens, i.e., one with a variable focal length. This principle has been applied to lenses used in spectacles by several inventors, most notably by Wright (U.S. Pat. No. 3,598,479).
A number of problems were encountered by workers attempting to fabricate variable focus spectacles using the principle of pumping liquid into a liquid lens. Among these were leakage, the difficulty of moving sufficient liquid in a short time, and annoying transient optical distortions caused by the streaming liquid.
Gordon (U.S. Pat. No. 1,269,422) disclosed radially squeezing the periphery of two abutting meniscus-shaped pieces of glass with liquid between them so as to cause the centers to bulge out, and cause a change in focal length.
Mitchell (British Patent No. 258,325) disclosed two different types of variable focal length lenses. Mitchell disclosed both pumping liquid into or out of the space between two sheets of transparent material, as previously disclosed by Ohmart, and also moving the peripheries of two spaced sheets closer or further apart, with the volume of liquid between them being fixed. The movement in this latter embodiment was accomplished by means of "a screw, lever or other suitable means". Since the volume of liquid between the sheets was fixed, moving them with respect to each other resulted in a change in curvature, and consequently, a change in the optical power or focal length of the lens.
Barnea (U.S. Pat. No. 4,913,536) also disclosed moving the peripheries of two transparent sheets closer or further apart, using a fixed volume of liquid. In addition, Barnea disclosed an actuating mechanism for the lens which involved pumping liquid (from a separate source) into or out of an "inner tube" occupying the space between the sheets at their peripheries. As the "inner tube" is inflated or deflated, the sheets move further apart or closer together, which results in a changing focal length.
The present inventors, in a copending application (Kurtin et al. U.S. patent application Ser. No. 08/226,344), have disclosed a liquid-filled variable focus lens in which a rigid lens and a distensible membrane (carried on a supporting structure) are positioned face to face and connected through a hinge. The space between the membrane and the rigid lens is filled with liquid, and a flexible seal around the periphery keeps the liquid contained. Angular motion of the membrane-carrying structure with respect to the rigid lens causes the membrane to change shape, thereby changing the focal length of the lens.